Never mind how grasshoppers hop. These engineers watch them fly

Grasshoppers may not spring to mind as paragons of graceful flight. But for a team of Princeton engineers, these gangly insects have inspired a new approach to robotic wings.

Typical designs for insect-scale flying robots draw inspiration from bees or flies, relying on constant flapping motion. That flapping draws a lot of power, and delivering that power is difficult because batteries are heavy, particularly for tiny robots. Grasshoppers add another technique to the mix. They don’t just flap, they also jump and glide.

“Gliding is a mode of cheap flight,” said Aimy Wissa, associate professor of mechanical and aerospace engineering and the study’s principal investigator. Her team is studying grasshoppers to build a glider that can fold its wings in and out and change strategies depending on the situation. “When we want to produce thrust, we flap. When we want to conserve energy, we fully deploy the wings and glide.”

By investigating how grasshoppers glide, the team has developed a model that could enable multimodal locomotion for tiny robots. This could give engineers new options in the quest to extend flight time for insect-sized robots.

The engineers teamed up with biologists to uncover the grasshopper’s secret to efficient gliding locomotion. They used those insights to 3D-print model wings between two and four inches wide. The paper was published in the Journal of the Royal Society Interface on Jan. 7.

Reducing the power needed to fly unlocks several possibilities for small, flying robots. Many existing models are tethered to wires to supply them with enough power. Using gliding flight as a part of their locomotion can reduce the power required to fly, allowing for a smaller battery payload and enabling untethered flight. Additionally, more power can be allocated to other forms of movement, such as crawling or jumping.

The project started with one question, according to the researchers: How can we design small robots that, like insects, move seamlessly across ground and air?

Only three insect groups can glide efficiently: grasshoppers, dragonflies and butterflies. While grasshoppers are not necessarily the best at gliding overall, they have the distinct ability to neatly retract their wings by folding them like an accordion.

Collapsing the wing allows for better mobility on the ground, according to Paul Lee, graduate student in mechanical and aerospace engineering and the paper’s lead author. “Dragonfly wings always stick straight out, and butterfly wings can only fold upward, which is limiting,” he said.

This feature also reduces air resistance when the grasshoppers jump from the ground to transition into flight, Wissa added.

When the researchers started deciding which features of the grasshopper’s wing to copy, they found that biology offered no clear answers about how each aspect of the wing impacts flight. “Then the research question flipped,” Wissa said. Instead of copying grasshoppers’ known biological design principles, they had to discover those principles themselves.

The team analyzed the wings of real grasshoppers to find out exactly how they fly so efficiently, collaborating with entomologists at the University of Illinois Urbana-Champaign. They focused on the American grasshopper, also called the bird grasshopper, because of its superior flying skills.

Grasshoppers have two sets of wings, the forewings and hindwings. The grasshopper’s hindwing is corrugated, meaning it has a 3D up-and-down pattern, like sharp hills and valleys, which allows it to fold in neatly. Biologists have long known that the front wing is mainly used for protection and camouflage, so the researchers studied the hindwing to find out how it’s used to combine flapping and gliding.

The researchers took CT scans, an imaging technique that uses X-rays and computing to capture the detailed geometry of an object, of real grasshopper wings in the Princeton Imaging Analysis Center.

They developed a new procedure for turning CT scans into 3D-printable designs, which they detail in the paper and could be useful for future studies of insect locomotion. They used their scans to 3D-print model wings with varying designs, incorporating different principles to test how each detail affects flight performance.

One of the strengths of using engineering to understand biology, according to Wissa, is that you can isolate different features — like a wing’s shape, camber, or corrugation — and test each one separately. “You can’t do that with an actual insect,” she said.

They tested the 3D-printed wings in a water channel, where they evaluated aerodynamic performance based on how water flows around the wing. Then they used the data gathered to further improve the design. They printed new wings and attached them to small frames to create realistic grasshopper-inspired gliders

Finally, they conducted flight experiments where they launched the gliders across the Princeton Robotics lab and used an advanced motion-capture camera system to evaluate flight performance. Using biological benchmarks for comparison, they found that the glider’s performance was on par with that of actual grasshoppers.

They also compared their grasshopper-inspired wing with another standard wing design. The standard design resembled a smooth version of the grasshopper hindwing and revealed insights into how the corrugations impacted performance. The smooth wing allowed the glider to fly more efficiently and more repeatably than one with the natural corrugations.

 “This showed us that these corrugations might have evolved for other reasons,” Wissa said. For example, the tests showed corrugations may help with flying at steep angles. Wissa said this study is a prime example of how engineering can contribute to biology, and vice versa.

The best-performing gliders they tested were smooth rather than corrugated, but future research could shed light on how to incorporate the corrugations to enable wing folding while still maximizing gliding efficiency.

The researchers are continuing to study grasshoppers to find ways to enable the new wings to deploy from a stowed position while minimizing the need for additional motors or power sources. “Very little is known about how grasshoppers deploy their wings," Wissa said. Unlocking this can greatly conserve power and reduce the size and cost of flying insect robots.

Wissa said another next step is to couple the design with jumping abilities. “If you're able to switch from crawling to jumping, then you can scale obstacles that are much bigger than your size,” Wissa said.

“This grasshopper research opens up new possibilities not only for flight, but also for multimodal locomotion,” Lee said. “By combining biology with engineering, we’re able to build and ideate on something completely new.”

The paper “From Grasshoppers to Gliders: Evaluating the Role of Hindwing Morphology in Gliding Flight” was published in the Journal of the Royal Society Interface Jan. 7. Besides Lee and Wissa, authors include Diaa Zekry, Ahmed K. Othman and Marianne Alleyne. The research was partially supported by the NSF CAREER Award and Princeton’s Addy Fund for Excellence in Engineering. The grasshopper specimens were provided by Scott Kirkton in the department of biology at Union College, and the imaging data were acquired with support from Princeton’s Imaging and Analysis Center, and the motion tracking and flight testing were conducted with support from Princeton’s Robotics Lab. 

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